A global navigation satellite system (GNSS) is a system of space-based satellites that provide autonomous geo-spatial positioning with global coverage. Generally, a GNSS allows receivers to determine their location using time signals transmitted along a line-of-sight from the satellites. The Global Positioning System (GPS) is a GNSS that is maintained by the United States government and can be used by anyone with a GPS receiver. Similarly, GLONASS is a navigation satellite system maintained by Russia. The Galileo system is another GNSS that is currently being built by the European Union (EU) and European Space Agency (ESA). COMPASS is a navigation satellite system being developed by China.
A GNSS provides location information anywhere on or near the Earth where there is an unobstructed line of sight to four or more GNSS satellites. A processor coupled to the GNSS receiver uses at least four of the distances from the receiver to the satellites, known as pseudoranges, to accurately approximate the position of the receiver. The accuracy of the approximate position, or position solution, varies as changing atmospheric conditions affect signal-to-noise ratios and signal transit times. The accuracy also varies as the orbiting satellites occasionally experience protracted failures during which they continue to operate while providing erroneous or extra-noisy signals. These and other factors appear as random noise in the transmitted signals, random errors in the computed pseudoranges, and ultimately as a random error in the position solution itself.
Aircraft navigation systems based on the GNSS typically include a subsystem for integrity monitoring. Integrity is a measure of the trust that can be placed in the correctness of the information supplied by the navigation system. Integrity includes the ability of a system to provide timely (within limits specified by the time-to-alert) and valid warnings to the user when the system must not be used for the intended operation (or phase of flight). The subsystem, known as a receiver autonomous integrity monitoring (RAIM), determines the horizontal protection level (HPL). The HPL is the radius of a circle in the horizontal plane which describes the region that is assured to contain the indicated horizontal position. The HPL is a horizontal region for which the missed alert and false alert requirements are met. The HPL is a function of the satellite and user geometry and the expected error characteristics and thus is not affected by actual measurements. Depending on the phase of flight, the vertical protection level may also need to be computed. For the integrity to be considered available for a particular phase of flight, the protection level shall be less than the alert limit specified for that phase of flight. If the protection level exceeds the allowed alert limit, this means the navigation system is not able to provide the required integrity and must not be used further.
Currently, integrity monitoring techniques are an important component of current GNSS based navigation systems designed to support non-precision phases of flight, for example, en-route or terminal phases, where only the requirements for the horizontal protection limits need to be met. With the expected modernization of GPS and GLONASS, as well as development of new constellations such as Galileo and COMPASS, there is interest in whether a navigation system taking advantage of the increased number of visible satellites, transmitting in more than one frequency band, will be able to support more demanding phases of flight, e.g., precision approaches. For these phases of flight, integrity of horizontal and vertical position components of the navigation information need to be monitored simultaneously.
Preliminary assessment of integrity for more demanding phases is a focal point of many recent studies. It has been shown that currently used integrity monitoring techniques do not provide sufficient performance for future intended applications. Therefore, in the GNSS Evolutionary Architecture Study (GEAS)—Phase II, February 2010 (hereafter “GEAS-Phase II”), an integrity monitoring technique called advanced RAIM (ARAIM) was proposed for “snap-shot” solution and vertical guidance. ARAIM is expected to be an important component of future navigation systems based on multi-constellation/multi-frequency GNSS receivers.